Every year the Boston Marathon attracts thousands of spectators who cheer on runners, but this year thousands more eyes – some digital and some undercover – will be watching intently as well, looking for anything that might warn them of a repeat of last year’s tragedy.

Authorities in charge of security for the marathon Monday are taking no chances, with an unprecedented security apparatus that includes some 4,000 deployed police officers, including 500 plainclothes officers, more than 100 surveillance cameras and a underground, futuristic coordination center boasting some 260 security officials representing more than 60 local, state and federal agencies.

“A lot of eyes,” Kurt Schwartz, Massachusetts’s Undersecretary for Homeland Security and Emergency Management in the Executive Office of Public Safety and Security, told ABC News. “There’s cameras, but you know there’s 4,000 police officers out there and they will be very engaged this year. They’re all watching the public, watching the crowds, trying to detect suspicious behavior, trying to manage areas that just get too crowded… We have expanded across the board.”

Schwartz and other top intelligence officials said there have been no specific threats made against the marathon. Then again, there weren’t any last year before two brothers from Dagestan allegedly detonated two bombs near the finish line, killing three people, including an 8-year-old boy, and injuring some 260 others.

So starting at 6 a.m. Monday inside the coordination center, which ABC News toured this weekend, dozens of officers and analysts will be monitoring wide-screen video displays relaying images from along the marathon route. Schwartz said that unlike last year, every section of the 26.2 mile race will be monitored by high-resolution cameras.

Nicknamed “the bunker,” the Cold War-era underground facility has no windows and is barren in military-style, save for the high-tech monitors, laptops and computer servers, with separate rooms for tactical, intelligence and other units. In addition to the surveillance capability, the center will be communicating in real-time with other offices across eight nearby cities and towns along the marathon path.

“We’ll be looking for somebody who just doesn’t feel right,” said Boston’s new police commissioner Bill Evans. “The characteristics – a lot of our officers, during their training, [are] looking at the characteristics of someone who might be carrying explosives.”

Beyond just watching, Schwartz said security officials will be tailoring their tactical security on the ground throughout the day of the marathon based on what the surveillance cameras and officers on the scene are seeing.

Evans told ABC News that though security will be tight, it won’t be overwhelming for runners or attendees.

“I don’t want it to be an armed camp where people are going to be intimidated by the police presence,” he said.

Evans’ men got a trial run last week when an alleged hoaxer dropped two bags near the finish line of the marathon, in a similar manner to how the real explosives were planted last year. Authorities reacted quickly and destroyed the ultimately harmless objects.

“It was a nice drill,” Evans said. “It just got us on our toes a little earlier… But I think we did a super job. We did what we were trained to do.”

In the case of last year’s bombing, authorities suspect it was carried out by Tamerlan and Dzhokhar Tsarnaev, two brothers from Dagestan who lived in the U.S. Tamerlan was killed in a shootout with police days after the explosions and Dzhokhar, his little brother, was arrested and has pleaded not guilty to 30 counts related to the bombing. If convicted, he could face the death penalty.

A full 36,000 runners signed up to race this year, undeterred by last year’s tragedy, including last year’s winners Lelisa Desisa from Ethiopia and Rita Jeptoo from Kenya.

“So I came here to support the whole injured and all the families [of the] lost ones,” Jeptoo told ABC News. “I’m not scared because I know there is security.”